1. Field of the Invention
The invention involves an aircraft landing system which will operate under zero visibility conditions. More specifically, the aircraft landing system provides the pilot a perspective view of a runway on which he intends to land.
2. Description of the Prior Art
It is essential that a pilot have an accurate aircraft landing system for use under conditions of zero visibility. Several types of systems are presently employed or proposed. One type of system uses an expensive surveillance radar to obtain a perspective view of the runway. This system requires a relatively large antenna to obtain the required azimuth resolution and a surveillance radar with a powerful short pulse transmitter, receiver and special display modes. Another type system uses radars and computers to guide the pilot in via radio links. An example is a Microwave Landing System (MLS). The MLS system is very expensive and complicated and cannot land an aircraft in zero-zero visibility conditions in the presence of winds across the runway. A third type of system involves the display of information derived fron Visual-Omni-Range (VOR) receivers, gyros, and other equipment of this type. This last type of system is not as accurate as desired.
A fourth type of system is proposed in U.S. Pat. No. 3,237,195 to J. Schiffman. This system uses microwave beacons arranged on both sides of a runway which are triggered to insure that no two beacons transmit simultaneously. To receive these signals the aircraft uses a 3-channel hybrid, phase amplitude-comparison monopulse radar which includes a pattern of 4 antennas with different phase centers and different boresight axes with two in the vertical plane and two in the horizontal plane. The sum of the signals from the four antennas unblanks a CRT beam when beacon signals arrive. The monopulse radar uses the difference of the signals received by the horizontally oriented antennas to determine the azimuth angle between the beacon and the average boresight axis of the pair of antennas. A voltage proportional to the azimuth angle is used to control the azimuth (x axis) of a CRT display. The vertically oriented antennas in a similiar manner determine the elevation angle of the beacons from the average boresight axis of the two antennas. A voltage proportional to the elevation angle is fed to the Y axis of the CRT. The use of the azimuth information of the X axis, the elevation information on the Y axis, and the sum signal on the Z (intensity) axis provides a perspective view of the runway much the same as a pilot would view with his eyes.
The problem with this system lies in the use of elevation angle information on the Y axis to establish a perspective view. One deficiency is that for each ground beacon there is a transmitted signal and a multipath transmitted signal (reflected off the ground) which are both received by the monopulse receiver. The multipath signal causes the error in the elevation angle determination and hence distorts the display presentation. In addition, as the aircraft gets closer to or over the runway and all of the elevation angles of the beacons begin to get very large the perspective view begins to move down on the display.